Many heat transfer systems use hot fluids as a heat transfer medium. Such systems include a heat generator for generating heat, a heat transfer medium in thermal communication with the energy source, and a pump to move the heated medium to wherever the heat is needed. Because of its high heat capacity and its abundance, a common heat transfer fluid is water, both in its liquid and gas phase.
A variety of heat generators are in common use. For instance, in nuclear power plants, nuclear fission provides energy for heating water. There also exist solar water heaters that use solar energy. However, most heat transfer sources rely on an exothermal chemical reaction, and in particular, on combustion of some fuel.